1. Field of the Invention
The present invention relates to a clamping mechanism of a rotation table device that disposes a rotation table device used in a machining tool in a stationary state.
2. Description of the Related Art
Heretofore, a worm gear mechanism having an in-reversible rotation characteristic has generally been employed as a rotation driving mechanism of a rotation table device (see, for example, Japanese Patent Application Laid-Open No. 2002-18678). However, in the case of the worm gear structure, a worm and a worm wheel mesh each other and a slight backlash exists. For that reason, even when a rotation table is held in a stationary state, rattling occurs by the amount corresponding to the backlash and the angle of the rotation table indexed cannot have a desirable value. As a result, it is difficult to achieve a high machining accuracy for a workpiece held on the rotation table.
Therefore, a clamping mechanism is used to keep the rotation table device at a stationary state when the rotation table device is stopped. For example, Japanese Patent Application Laid-Open No. 2012-202484 discloses a rotation table device provided with a brake disk which is fixed to a shaft transmitting a rotational movement from a rotational driving device of a direct drive structure to a table and rotates integral with the shaft and a piston which is provided so that the brake disk can be sandwiched between a fixed member and the piston. Then, when an air pressure is applied to the piston so that the brake disk is sandwiched between the piston and the fixed member, the movement of the shaft integrated with the brake disk is clamped by a friction force generated between the brake disk and the fixed member so that the rotation table device connected to the shaft is kept in a stationary state.
In this way, in the rotation table device of the rotational driving device of the direct drive structure, a mechanism for disposing the table in a stationary state does not exist other than a motor generating power. For this reason, the clamping mechanism is needed.
In a mechanism for pressing the brake disk against the fixed member to clamp the brake disk, generally, an urging mechanism is provided to urge the brake disk in a clamping or unclamping direction even in a state where air pressure or hydraulic pressure is not applied. In many cases, a plate spring is used as the urging mechanism. Here, the plate spring includes two functions; that is, a function of generating an urging force in a clamping or unclamping direction by a return of the spring due to elastic deformation in a state where air pressure or hydraulic pressure is not applied, and a function of increasing a clamping torque during a clamping operation by the piston. Here, the plate spring needs to be fixed to the piston and a rear plate of the rotation table device by the use of a bolt or the like. However, since the plate spring is bent during the clamping/unclamping operation and a rotation torque is added during the clamping operation, stress concentrates on the fixing portion and thus the fixing portion tends to be easily cracked due to aging.